Field of Invention
The invention relates to systems and materials used for solid-state thermodynamic heat pump cycles or refrigeration cycles. More particularly, the invention relates to solid-state thermodynamic heat pump cycles or refrigeration cycles based on thermoelastic effect.
Description of the Related Art
According to 2008 Buildings Energy Data, building space cooling and commercial and residential refrigeration will consume 7.46 quads of primary electricity and generate 447 million metric tons (MMT) of CO2 emission in 2030. (Buildings Energy Data Book, 2009, at Table 1.1.7 and Table 1.4.5). This is equivalent to ˜5% of primary energy consumption and ˜5% of CO2 emissions in US. Currently, more than 90% of space cooling in the US is provided by vapor compression (VC) based systems. (D. Westphalen and S. Koszalinski, Energy Consumption Characteristics of Commercial Building HVAC Systems, Vol. 1: Chillers, Refrigerant Compressors and Heating Systems; Arthur D. Little, Report For Office of Building Technology State and Community Programs, Department of Energy). Refrigerants used in VC systems are significant sources of greenhouse gas (GHG) emissions. Refrigerants such as hydrochlorofluorocarbons (HCFC) or halofluorocarbons (HFC) have global warming potential (GWP) as high as 1000 times that of CO2 (See Buildings Energy Data Book, 2009). As such, there is an urgent need to develop a new and affordable cooling technology, which enhances overall energy efficiencies and reduces GHG emission in space cooling and refrigeration.
In addition to building space cooling and refrigeration, transport space cooling and refrigeration, and instrument temperature control also need new and affordable cooling technology, which enhances overall energy efficiencies and reduces GHG emissions.
There exist a number of refrigeration technologies. Currently, vapor compression is the dominant technology. More than 90% of cooling is provided by vapor compression based systems in U.S. (See D. Westphalen and S. Koszalinski, Energy Consumption Characteristics of Commercial Building HVAC Systems, Vol. 1, supra). A new technology that is more energy efficient and environmentally friendly is urgently needed to replace the vapor compression technology. Candidate technologies include electrocaloric, magnetocaloric, thermoacoustic, thermoelectric and thermoelastic. Table 1 briefly compares these cooling technologies.
TABLE 1Comparison of various cooling technologies.OverallSystemEnvironmentalTechnologyPrincipleCOPImpactCostReferenceVaporVaporization4HighLowV. Pecharsky, K. Gschneider, Jr.,compressionlatentPRL 78, 4494 (1997)heatElectrocaloricElectrocaloric—LowHighY. V. Sinyavskii, Chem. and Petrol.effectEng., Vol. 31, p. 295, n5-6 (1995);A. S. Mischenko, Q. Zhang et. al.,Science, Vol. 311, pp. 1270-71,n5765 (2006); Neese, Chu, et al.,Science, Vol. 321, p. 821, n5890(2008)MagnetocaloricMagnetocaloric15LowHighK. Gschneider, V. Pecharsky, Annu.effectRev. Mater. Sci., 2000, v. 30, pp.387-429; J. L. Hall, J. A. Barclay,Advance Cryo. Eng., Vol. 43, pp.1719-1728 (1998); K. Gschneider,et al., Proceedings 50th AnnualInt'l Applicance Tech., pp. 144-154(1999)Thermo-Ideal gas0.8LowMedL. Garrett, Am. J. Phys. Vol. 72,acousticlawpp. 11-17, n.1 (2004); A. Bejan,Adva. Eng. Thermodynamics(Wiley, N. Y. 2nd ed., 1997);S. Backhaus, G. W. Swift, Nature,Vol. 399, pp. 335-338 (1999)Thermo-Peltier0.7LowMedTE Technology, Inc.,electriceffecthttp://www.tetech.com/techinfo;D. S. Kim, C. A. Infante Ferreira,Int'l J. Refg., Vol. 31, pp. 3-15, n.1 (2008); G. J. Snyder, T. S. Ursell,Physical Review Letters, Vol.91, pp. 148301-4, n. 14 (2003)Thermo-Martensitic12.5LowLowP. H. Leo, T. W. Shield, O. P. Bruno,elasticphaseActa Metall. Materil., Vol.transformation41, No. 8, pp. 2477-2485 (1993)latentheat
Vapor-compression refrigeration has been and still is the most widely used method for air-conditioners and refrigerators. The method relies on latent heat released or absorbed during pressure induced gas-liquid transition. Since its invention in 1805 by Oliver Evans, the efficiency of this technology has been significantly improved. Compared to the refrigerator built in 1970's, current Energy Star rated refrigerators use nearly 3 times less electricity. The compressor is frequently the first target for manufacturers looking to improve power consumption in their products. As a result of decades of effort, current compressors are highly efficient (˜60%) and cost-effective. Adding other system improvements such as seals, valves, muffler, heat exchangers, and thermal insulation, a modern refrigerator can be as efficient as 45%. However, to achieve more than incremental gain in efficiency, a fundamental change must be explored. In addition to the efficiency plateau, vapor-compression technology also faces adverse environmental circumstances due to its dependence on hydrochlorofluorocarbons or halofluorocarbons refrigerants, of which the global warming potential is typically more than 1000 times that of CO2. (See http://www.whitehouse.gov/administration/eop/nec/StrategyforAmericanInnovation; see also Buildings Energy Data Book, 2009). Even though the cost of manufacturing vapor-compression based air-conditioners and refrigerators is low, the efficiency limit and environmental issues make this technology undesirable.
Electrocaloric effect is not commercially exploited as the effect is insufficient for practical application. Recently, the technology received renewed interest because of two papers published in the Science journal, both of which demonstrated a giant electrocaloric effect. (A. S. Mischenko, Q. Zhang et al., Science, Vol. 311, pp. 1270-71, n5765 (2006); Neese, Chu, et al., Science, Vol. 321, p. 821, n5890 (2008)). In one paper, Mischenko shows that a thin film Pb(Zr0.95Ti0.05)O3 exhibits a ΔT of 12 K and ΔS of 8 J/(kg-K) with electric field of 480 kV/cm at 499 K. In the other paper, Neese shows that copolymer P(VDF-TrFE) film exhibits a ΔT of 12 K and ΔS of 55 J/(kg-K) with electric field of 300 kV/cm at 343K. While these findings are exciting and have the potential to open a new field of research, their commercial potential remains low because thin film forms (350 nm for the ceramic and 2000 nm for the co-polymer) present limitation on cooling capacity, and low thermal conductivity of the ceramic and polymer materials seriously impedes heat exchange efficiency. Various designs had been proposed to overcome the capacity limitation. But given the characteristic of the device, this technology is more suitable for spot cooling where space is at premium and high efficiency is secondary.
Magnetocaloric refrigeration has received substantial interest recently. The number of research papers in this area has increased exponentially in the past decade. Magnetocaloric refrigeration relies on the magnetocaloric effect, where reversible temperature change is caused by the application and removal of a magnetic field. The effect was discovered by Emil Warburg in 1881. Its theoretical Carnot efficiency approaches 68%. But, efforts to commercialize current magnetic refrigeration technology have been ineffectual due to the high cost implicated by its fundamental requirement of large quantity of rare-earth elements for working materials and for the magnetic field. Researchers have investigated the commercial viability of the current magnetic refrigeration technology, and concluded that a minimum 10× reduction in cost is required to achieve commercial feasibility. Most researchers are aware of the cost issues. Their focus has been on materials development, either trying to find new materials containing less expensive elements, or trying to increase the cooling capacity to improve the dollar/watt ratio. Fundamental research efforts typically do not address the cost issue associated with the requirement of large magnetic field. This is because the physics of magnetocaloric effect dictates that the cooling capacity is proportional to the applied magnetic field. It is estimated that for every 1 kg of rare earth based refrigerant materials, 6 kg of rare earth based permanent magnet is needed to supply the minimum required magnetic field. Hence, the strong dependence on rare earth materials limits the commercial viability of this technology.
Thermoacoustic refrigeration can be traced back to 1887, when Lord Rayleigh discussed the possibility of pumping heat with sound. The method relies on the ideal gas law, where high amplitude sound waves are applied to a pressurized gas to pump heat. The most efficient device built to date has an efficiency approaching 40% of the Carnot limit, or about 20˜30% of the overall system efficiency. Despite significant progress achieved in the past decades, the commercial viability of the current thermoacoustic technology remains low.
Thermoelectric refrigeration is commonly used in camping and portable coolers and for cooling electronic component and small instruments. The method relies on the Peltier effect, the caloric effect of an electrical current at the junction of two dissimilar metals. The thermoelectric effect was discovered by Jean-Charles Peltier in 1834. The efficiency of a thermoelectric junction is low from 5˜10%. Despite its compactness, applications are limited to small scales due to the low efficiency. The impact to the modern energy shortage and global warming is small.